System and method for hydrogen exfoliation

ABSTRACT

A system and method for hydrogen (H) exfoliation are provided for attaching silicon-on-insulator (SOI) fabricated circuits to carrier substrates. The method comprises: providing a SOI substrate, including a silicon (Si) active layer and buried oxide (BOX) layer overlying a Si substrate; forming a circuit in the Si active layer; forming a blocking mask over selected circuit areas; implanting H in the Si substrate; annealing; removing the blocking mask; in response to the H implanting, forming a cleaving plane in the Si substrate; bonding the circuit the top oxide layer to the carrier substrate; and, cleaving the Si substrate. More specifically, the cleaving plane is formed along a horizontal peak concentration (Rp) H layer in the Si substrate and along the buried oxide layer interface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) and liquidcrystal display (LCD) fabrication and, more particularly, to a methodfor the transfer of partially completed very-large-scale integration(VLSI) circuits from single crystal silicon substrates to glass panels,for the production of flat panel displays.

2. Description of the Related Art

There is broad agreement in the flat panel display (FPD) industry thatsystem-on-glass (SOG) technology is a natural evolutionary step for flatpanel displays, especially for mobile devices. In fact, SOG is a naturalconfluence of display and microprocessor evolution because integrationis a proven solution for greatly reducing costs, while improving thecompactness and reliability of electrical systems.

Display modules have received some enhanced functionality, like displaydrivers and analog-to-digital converters, thanks to low-temperaturepolysilicon (LTPS) technology. Conventionally, the best means forachieving adequate LTPS performance for SOG devices at a competitivecost is the crystallization of a thin amorphous layer of silicon with alaser beam. Unfortunately, this approach remains relatively expensive,even years after being introduced to LCD production. Moreover, even ifpolysilicon thin film transistors (TFTs) could be efficiently producedusing this method, it is unlikely that they will have sufficientcapability to realize some sophisticated functions like CPU operationsand digital signal processing. Finally, the steadiness of drive currentsproduced by poly-Si TFTs may be inadequate for new technologies suchorganic electroluminescent displays.

In short, the convolution between cost and performance of polysilicondevices still pales compared to that of single-crystal silicon (c-Si).The trouble is in creating quality thin films of c-Si on transparent orflexible substrates, which usually are made of materials other thansilicon. Other solutions to this hybrid field problem includeFluidic-Self-Assembly™ (FSA) by Alien Technology. FSA works fairly wellfor plastic substrates and semiconductor blocks thicker than 50 microns.Unfortunately, FSA placement depends on random probability and gravity.Because the probability of successful placement is small (<<20%), alarge amount of blocks are needed. Also, as the blocks get smaller,Brownian motion becomes more disruptive to precise placement and moretime is required for settlement. Finally, if glass substrates aredesired, then another problem is the efficient etching of precise-sizedholes. Other rapid assembly techniques, like capillary self-assembly,still require fluid, which usually demands use of surfactants, andremain susceptible to Brownian motion.

SOITEC and other researchers have developed and refined a means ofefficiently creating thin films of c-Si by ion-cutting with a high dosehydrogen implantation. In related work, Joly et al. have extended theion-cutting process (Smart-Cut) to produce devices on one substrate, andtransfer these devices to a different substrate. While their workdescribes a process for transferring the devices, there is littlediscussion regarding the impact of high dose hydrogen implantation ondevice performance. It is acknowledged by many that the required largedoses of hydrogen (˜5e¹⁶ atoms/cm²) can result in highly defectiveregions in the transferred silicon films.

FIG. 1 is a diagram of a hydrogen-induced cleaving process using ahydrogen blocking mask (prior art). To address some of the problemsassociated with the use of hydrogen, Bruel et al., describe the use of ablocking mask to protect active silicon regions from damage during thehydrogen implant. However, while blocking the channel areas during thehydrogen implant prevents damage, the subsequent transfer of the blockedregions (areas without H⁺ implant) is problematic. It is difficult tocleave the active Si regions from the underlying Si substrate in anyreliable or predictable manner. Therefore, this process has not provedto be practical for large-scale fabrication processes, or for thetransfer of very large active Si areas, such as VLSI circuits with aplurality of blocked areas.

It would be advantageous if a process existed for placing partiallycompleted VLSI crystalline silicon devices onto non-silicon substratesfor the purpose of making large area devices, especially displaysystems, such as display matrices, complementary signal processes, andcontrol circuitry.

It would be advantageous if the above-mentioned transfer processincluded the use of blocking masks to prevent damage to sensitive areasduring H+ implant, while permitting those areas to be transferred toanother substrate via wafer bonding and cleaving.

It would be advantageous if the heat load of above-mentioned transferprocess could be accommodated by plastic substrates, including PETmaterials.

SUMMARY OF THE INVENTION

This application describes a process for partially, or completelyfabricating single-crystalline devices on non-silicon, heat sensitivesubstrates. The method avoids the use of an intermediate substrate forthe transfer of devices, and avoids the need for post-transfer thinningof active silicon layers. The process provides a means for transferringdevices with more than 1 layer of metal. The process permits a blockingmask to be used, to prevent damage to active channel regions from H⁺implants. Further, the method permits high temperature processing to becompleted on silicon substrates and then transferred to low temperaturesubstrate such as glass or plastic.

Accordingly, a hydrogen (H) exfoliation method is provided for attachingsilicon-on-insulator (SOI) fabricated circuits to carrier substrates.The method comprises: providing a SOI substrate, including a silicon(Si) active layer and buried oxide (BOX) layer overlying a Si substrate;forming a circuit in the Si active layer, such as a TFT; forming ablocking mask over selected circuit areas, such as the TFT gate;implanting H in the Si substrate; annealing; removing the blocking mask;depositing a top oxide layer overlying the circuit; planarizing the topoxide layer; in response to the H implanting, forming a cleaving planein the Si substrate; bonding the circuit the top oxide layer to thecarrier substrate; and, cleaving the Si substrate.

More specifically, forming a cleaving plane in the Si substrate includesforming a horizontal peak concentration (Rp) H layer in a masknon-underlying area in the Si substrate. Then, cleaving the Si substrateincludes: cleaving a first region along the horizontal Rp layer in themask non-underlying area in the Si substrate; cleaving a second regionalong a horizontal interface between the buried oxide layer and Sisubstrate; and, cleaving vertically between the first and secondregions.

Using this process, the circuit in the Si active layer can be formed atprocess temperatures greater than some critical temperature, and thecircuit can be subsequently bonded to a carrier substrate, such as glassor plastic, that is sensitive to temperatures greater than the criticaltemperature.

Additional details of the above-described method and an active Si layercleaved from an SOI substrate are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a hydrogen-induced cleaving process using ahydrogen blocking mask (prior art).

FIG. 2 is a partial cross-sectional view of an active silicon (Si)device cleaved from a silicon-on-insulator (SOI) substrate.

FIGS. 3 through 10 are partial cross-sectional views depicting steps inthe fabrication of the active Si device of FIG. 2.

FIGS. 11 through 13 are plan and partial cross-sectional views providingsome additional process details.

FIG. 14 is a flowchart illustrating a hydrogen (H) exfoliation methodfor attaching silicon-on-insulator (SOI) fabricated circuits to carriersubstrates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a partial cross-sectional view of an active silicon (Si)device cleaved from a silicon-on-insulator (SOI) substrate. The activeSi device 200 comprises a Si substrate 202 with a cleaving plane surface204 and a bottom surface 206. A SOI substrate 208, including a Si activelayer 210 and buried oxide (BOX) layer 212 overlies the Si substratebottom surface 206. A circuit 214 is formed in the Si active layer 210.The active layer 210 can be single-crystal or polycrystalline Si. Inthis case, the circuit 214 is a thin-film transistor with a gate 216,source/drain (S/D) areas 218, and a channel region 220 adjacent the S/Dregions 218. With a TFT circuit 214, it is desirable that the hydrogen(H) used in the cleaving process be kept from diffusing into the channelregion 220, as it degrades transistor electrical performance. Although atransistor circuit is shown as an example, the cleaved structure hasapplication to any sensitive semiconductor region that must be kepthydrogen free. For example, the circuit 214 in the Si active layer 210may also be CMOS circuits, VLSI devices, or a TFT integrated into CMOSor VLSI devices.

The Si substrate cleaving plane surface includes a first thicknesssurface 204 a in areas underlying the Si active layer channel-adjacentareas, for example the S/D areas 218. A second thickness surface 204 b,where the second thickness 222 is less than the first thickness 224, isformed in areas underlying the Si active layer channel region 220.Vertical plane surfaces 204 c are formed between the areas of the firstand second thickness surfaces 204 a and 204 b. Also shown is a thirdthickness surface 204 d underlying the Si active layer field regions226, adjacent the S/D areas 218. Vertical plane surfaces 204 e areformed between the areas of the second and third thickness surfaces 204b and 204 d. As shown, the second thickness 222 is approximately zero.That is, the second thickness surface 204 b is formed on the surface 230of the BOX. In other aspects, the second thickness 222 is in the rangeof 0 to 1000 Å. The first thickness 224 is in the range of 20 to 1000nanometers (nm), depending on a variety of factors such as SOI thickness240, dosage, implanting species, and implantation times.

As explained in greater detail below, a H-blocking mask temporarilyoverlies the SI active layer channel region 220, and more directlyoverlies the gate 216 (see FIG. 5). The blocking mask prevents theincorporation of H into the channel region 220 during the implantationprocess. Alternately stated, the Si substrate first thickness surface204 a includes a peak concentration of hydrogen (H), while the secondthickness surface 204 b includes a minimum concentration of H, typicallynone (zero), see FIG. 5. The Si substrate first thickness surface 204 apeak concentration of hydrogen is in the range of 5×10¹⁵ to 5×10¹⁷.

In other aspects, the Si substrate first thickness surface 204 aincludes a peak concentration of an additional element such as boron He,Ne, Ar, or Si, while the second thickness surface 204 b includes aminimum concentration of the additional element, typically none (zero).In the case of boron, the Si active layer first thickness surface 204 aboron concentration is in the range of 5×10¹² to 5×10¹⁴ at/cm². In thecase of He, the Si substrate first thickness surface 204 a includes apeak concentration of H at a dosage in the range of 2×10¹⁶ at/cm² to3×10¹⁶ at/cm² and He at a dosage in the range of 1×10¹⁶ at/cm² to 3×10¹⁶at/cm². Again, the second thickness surface 204 b includes a minimum(typically zero) concentration of H and He.

A planarized oxide layer 240 overlies the active Si layer circuit 214. Acarrier substrate 242, made from a material such as glass, plastic,quartz, and metal foil is bonded to the oxide layer 240. Alternately,the carrier substrate can be any semiconductor film or material that issensitive to single crystal active circuit fabrication (hightemperature) processes. After bonding and cleaving the carrier substrate242, a planarization process is typically performed to remove the Sicleaving plane 204, down to the BOX surface 230. However, in someaspects the remaining Si is not removed.

Functional Description

FIGS. 3 through 10 are partial cross-sectional views depicting steps inthe fabrication of the active Si device of FIG. 2. In FIG. 3, theprocess begins with a SOI type wafer, including an active silicon layerof desired thickness and buried oxide layer of required thickness. InFIG. 4 the devices are fabricated in accordance to standard VLSIprocessing (i.e. CMOS, SOI, etc.) techniques. The conventional processesare stopped after activation of source/drain implants or a last hightemperature step. In FIG. 5, a blocking mask is patterned to protectchannel, and other regions, from hydrogen. Implantation is performedwith H⁺ or H₂ to place R^(p) (a maximum concentration) of hydrogenwithin the handle silicon substrate. In FIG. 6, processing continues tocomplete the metal layers. Typically, the temperature of deposition andsubsequent processing is kept less than the cleaving temperature of theH⁺ implanted wafer. A smoothing oxide layer is deposited and planarizedwith either chemical-mechanical polishing (CMP) or by usingspin-on-glass (SOG).

In FIG. 7, the bond wafer (or diced die) is attached to a non-siliconsubstrate. In FIG. 8, it can be seen that the implanted H+ establishes acleaving plane at the R^(p) location. When the bonded wafers (or die)are heated, cleaving results at the R^(p) location. Where there is no H⁺implant (i.e. under the channel) the cleaving will propagate verticallyuntil the buried oxide/silicon oxide layer is reached. Then, thecleaving will proceed laterally at the BOX interface. In FIG. 9, a hightemperature annealing process is performed to cleave the handle siliconwafer. In FIG. 10, the remaining silicon is removed using standardreactive ion etch techniques, for example.

The above-described fabrication process uses silicon-on-insulator wafersimplanted with H+. For example, this process permits thin filmtransistors to be fabricated on SIMOX substrates. Photoresist ispatterned to block the channel areas of these devices from subsequent H+implant according to the method described above. After H+ implant, TEOSoxide is deposited and planarized, and the SIMOX wafer (with the TFT's)is bonded to a glass substrate. The bonded pair is then heated to 600°C. to induce cleaving.

FIGS. 11 through 13 are plan and partial cross-sectional views providingsome additional process details. In FIG. 11, H+ is implanted into a SOIwafer using a blocking mask to protect the channel area. The bonded pair(SOI substrate/glass substrate) is heated to induce cleaving along H+layer and buried oxide layer. FIG. 12 shows the cleaving profile, asmeasured via profilometry, after cleaving. The step height of theunderlying silicon layer corresponds with that predicted by simulation(849 nm). In FIG. 13, the top transferred Si layer is etched to removeit. After removing the top silicon layer, the channel region and theburied device layer is now visible. There is a slight depression in theoxide layer due to the silicon etch step that also slightly etchesoxide. Note that a silicon layer free of H+ implant damage is obtained,sandwiched by thermal oxide. The structure yields higher performance, aswell as greater stability and reliability.

In other aspects (see FIG. 6), the fabrication of the devices can bestopped at gate metal deposition. Instead of producing the first layerof metal, the devices can be transferred to the non-silicon substratewith only the gate metal and source/drain activation completed.

In another variation, the implant step can include a boron implant tothe same depth as the hydrogen implant. The addition of implanted boroncan induce breakage at a much lower temperature, often 200° C. to 300°C., which allows a wider choice of substrates and metals. Other speciescan be co-implanted to improve the cleaving performance. These otherspecies include He, Ne, Ar, or Si.

In a different aspect, the cleaving step need not be thermally induced.If the bonding strength is stronger than the required cleaving force,the cleaving could be induced by mechanical methods.

In another aspect, the etching process discussed in the explanation ofFIG. 10 can be eliminated. Instead, the remaining silicon can be used asa protective layer for subsequent processing. More specifically, in thecase of blanket laser crystallization, this silicon layer can act as athermal sink to prevent dopant redistribution in the source/drainregions during laser crystallization. The buried oxide layer of theoriginal SOI material can be utilized as a basecoat material forsubsequent polysilicon processing of hybrid circuits.

In one aspect, instead of using the buried oxide layer in the originalSOI substrate, the SOI substrate can be fabricated in an alternativestack layer that allows for easier cleaving. For example, a porous Silayer can be used instead of SOI.

In a different aspect (see FIG. 9), the top silicon layer remaining onthe BOX after cleaving can be used as an active device layer forsubsequent device fabrication. This structure could be used for 3-Ddevice integration. Further, the overall transfer process can berepeated (i.e. device fabrication on single crystal, then transfer todifferent substrate) to yield 3-D device integration.

FIG. 14 is a flowchart illustrating a hydrogen (H) exfoliation methodfor attaching silicon-on-insulator (SOI) fabricated circuits to carriersubstrates. Although the method is depicted as a sequence of numberedsteps for clarity, no order should be inferred from the numbering unlessexplicitly stated. It should be understood that some of these steps maybe skipped, performed in parallel, or performed without the requirementof maintaining a strict order of sequence. The method starts at Step300.

Step 302 provides a SOI substrate, including a silicon (Si) active layerand buried oxide (BOX) layer overlying a Si substrate. Step 304 forms acircuit in the Si active layer. For example, TFTs, CMOS circuits, orVLSI devices may be formed. Step 306 forms a blocking mask over selectedcircuit areas. For example, the mask can be a photoresist material. Step308 implants H in the Si substrate. Step 312 forms a cleaving plane inthe Si substrate in response to the H implanting. Alternately stated,Step 306 forms a blocking mask over selected areas of the Si substrate,and Step 312 prevents the formation of H in the selected areas of the Sisubstrate, in response to the blocking mask. Step 314 bonds the circuitto a carrier substrate. Step 316 cleaves the Si substrate. In oneaspect, Step 318 ion etches, following the cleaving of the Si substrate,and Step 320 removes any remaining Si substrate material.

In one aspect, forming a cleaving plane in the Si substrate in Step 312includes forming a horizontal peak concentration (Rp) H layer in a masknon-underlying area in the Si substrate. Then, cleaving the Si substratein Step 316 includes substeps. Step 316 a cleaves a first region alongthe horizontal Rp layer in the mask non-underlying area in the Sisubstrate. Step 316 b cleaves a second region along a horizontalinterface between the buried oxide layer and Si substrate. Step 316 ccleaves vertically between the first and second regions. The terms“horizontal” and “vertical” are relative and only used for convenience.

In one aspect, forming a circuit in the Si active layer (Step 304)includes forming a thin-film transistor (TFT). Then, forming theblocking mask over selected circuit areas in Step 306 includes formingthe blocking mask overlying a TFT gate.

In a different aspect, Step 304 forms the circuit at processtemperatures greater than a first temperature. Then, bonding the circuitto a carrier substrate in Step 314 includes bonding to a carriersubstrate sensitive to temperatures greater than the first temperature.For example, the carrier substrate may be glass, plastic, quartz, metalfoil, or a carrier fabricated with components sensitive to temperaturesgreater than the first temperature. In one aspect, Step 304 forms thecircuit at a (first) temperature greater than 600 degrees C.

In one variation, implanting H in the Si substrate (Step 308) includessubsteps (not shown). Step 308 a implants H in a form such as H+ or H2.Step 308 b implants (H) at a maximum energy of 1 MeV. Step 308 cimplants at a dosage in the range of 5×10¹⁶ to 5×10¹⁷.

In another aspect Step 309 a (not shown), following the H implanting,removes the blocking mask. Step 309 b (not shown) deposits a top oxidelayer overlying the circuit. Step 309 c (not shown) planarizes the topoxide layer. Then, bonding the circuit to a carrier substrate in Step314 includes directly bonding the top oxide layer to the carriersubstrate.

Typically, the method includes the additional step (Step 310) ofannealing the Si substrate. Then, the Si substrate is cleaved (Step 316)in response to the annealing. In one aspect, the annealing temperatureis in the range of 400 to 600 degrees C. In another aspect, Step 316cleaves the Si substrate in response to a mechanical separation process.The mechanical separation process may additionally include the annealingstep.

In another aspect, implanting H in the Si substrate (Step 308) mayinclude additional implanting a species such as boron, He, Ne, Ar, orSi. For example, boron can be implanted at a dosage in the range of5×10¹² to 5×10¹⁴ at/cm². Then, Step 310 anneals the Si substrate at atemperature in the range of 200 to 300 degrees C. In another example, His implanted at a dosage in the range of 2×10¹⁶ at/cm² to 3×10¹⁶ at/cm².In addition, He is implanted at a dosage in the range of 1×10¹⁶ at/cm²to 3×10¹⁶ at/cm².

A method for cleaving active Si circuitry from a Si substrate, forattachment to a temperature sensitive substrate, has been presented.Details of particular materials, temperatures, and implantation specieshave been provided to illustrate the invention. However, the inventionis not limited to merely these examples. Other variations andembodiments of the invention will occur to those skilled in the art.

1. A hydrogen (H) exfoliation method for attaching silicon-on-insulator(SOI) substrate fabricated circuits to carrier substrates, the methodcomprising: providing a SOI substrate, including a silicon (Si) activelayer and buried oxide (BOX) layer overlying a Si substrate; forming acircuit in the Si active layer; forming a blocking mask over selectedcircuit areas; implanting H in the Si substrate; in response to the Himplanting, forming a cleaving plane in the Si substrate; bonding thecircuit to a carrier substrate; cleaving the Si substrate: whereinforming a cleaving plane in the Si substrate includes forming ahorizontal peak concentration (Rp) H layer in a mask non-underlying areain the Si substrate; and, wherein cleaving the Si substrate includes:cleaving a first region along the horizontal Rp layer in the masknon-underlying area in the Si substrate; cleaving a second region alonga horizontal interface between the buried oxide layer and Si substrate;and, cleaving vertically between the first and second regions.
 2. Themethod of claim 1 wherein forming a circuit in the Si active layerincludes forming a thin-film transistor (TFT); and, wherein forming ablocking mask over selected circuit areas includes forming the blockingmask overlying a TFT gate.
 3. The method of claim 1 wherein forming acircuit in the Si active layer includes forming a circuit at processtemperatures greater than a first temperature; and, wherein bonding thecircuit to a carrier substrate includes bonding to a carrier substratesensitive to temperatures greater than the first temperature.
 4. Themethod of claim 3 wherein bonding to a carrier substrate sensitive totemperatures greater than the first temperature includes bonding to acarrier substrate selected from the group including glass, plastic,quartz, metal foil, and a carrier fabricated with components sensitiveto temperatures greater than the first temperature.
 5. The method ofclaim 4 wherein forming a circuit at process temperatures greater than afirst temperature includes forming a circuit at a temperature greaterthan 600 degrees C.
 6. The method of claim 1 wherein implanting H in theSi substrate includes: implanting H in a form selected from the groupincluding H+ and H2; implanting at a maximum energy of 1 MeV; andimplanting at a dosage in the range of 5×10¹⁵ to 5×10¹⁷.
 7. The methodof claim 1 further comprising: following the H implanting, removing theblocking mask; depositing a top oxide layer overlying the circuit;planarizing the top oxide layer; and wherein bonding the circuit to acarrier substrate includes directly bonding the top oxide layer to thecarrier substrate.
 8. The method of claim 1 further comprising:annealing the Si substrate; and, wherein cleaving the Si substrateincludes cleaving in response to the annealing.
 9. The method of claim 8wherein annealing the Si substrate includes annealing at a temperaturein the range of 400 to 600 degrees C.
 10. The method of claim 8 whereinimplanting H in the Si substrate additionally includes implanting boronat a dosage in the range of 5×10¹² to 5×10¹⁴ at/cm²; and, whereinannealing the Si substrate includes annealing at a temperature in therange of 200 to 300 degrees C.
 11. The method of claim 1 whereincleaving the Si substrate includes cleaving in response to a mechanicalseparation process.
 12. The method of claim 1 wherein forming a blockingmask over selected circuit areas includes forming a mask from aphotoresist material.
 13. The method of claim 1 wherein implanting H inthe Si substrate additionally includes implanting a species selectedfrom the group including boron, He, Ne, Ar, and Si.
 14. The method ofclaim 1 further comprising: following the cleaving of the Si substrate,ion etching; and, removing any remaining Si substrate material.
 15. Themethod of claim 1 wherein forming a circuit in the Si active layerincludes forming a circuit selected from the group including TFTs, CMOScircuits, and VLSI devices.
 16. The method of claim 1 wherein implantingH in the Si substrate includes: implanting H at a dosage in the range of2×10¹⁶ at/cm² to 3×10¹⁶ at/cm²; and, additionally implanting He at adosage in the range of 1×10¹⁶ at/cm² to 3×10¹⁶ at/cm^(2.)
 17. The methodof claim 1 wherein forming a blocking mask over selected circuit areasincludes forming a blocking mask over selected areas of the Sisubstrate; and, the method further comprising: in response to theblocking mask, preventing the formation of H in the selected areas ofthe Si substrate.